This invention relates to a novel block copolymer composition for use in low viscosity low temperature hot melt adhesives. More particularly, it relates to predominantly branched styrene-isoprene-styrene block copolymer compositions comprised of linear polymeric blocks and adhesives made using such compositions.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise primarily those having a general structure EQU A--B and A--B--A
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, while block B is a polymer block of a conjugated diene such as polyisoprene. The proportion of the thermoplastic blocks to the elastomeric polymer block and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics. When the content of the alkenyl arene is small, the produced block copolmer is a so-called thermoplastic rubber. In such a rubber, the blocks A are thermodynamically incompatible with the blocks B resulting in a rubber consisting of two phases--a continuous elastomeric phase (blocks B) and aabasically discontinuous hard, glass-like plastic phase (blocks A) called domains. Since the A--B--A block copolymers have two A blocks separated by a B block, domain formation results in effectively locking the B blocks and their inherent entanglements in place by the A blocks and forming a network structure.
These domains act as physical crosslinks anchoring the ends of many block copolymer chains. Such a phenomena allows the A--B--A rubber to behave like a conventionally vulcanized rubber in the unvulcanized state and is applicable for various uses. For example, these network forming polymers are applicable for uses such in adhesive formulations; as moldings of shoe soles, etc.; impact modifier for polystyrene resins and engineering thermoplastics; modification of asphalt; etc.
Styrenic block copolymers are widely used to make hot melt adhesives for a wide variety of uses including diaper assembly. Hot melt adhesives based on styrenic block copolymers such as KRATON.RTM. thermoplastic rubber provide excellent adhesion to non-polar substrates such as polethylene which are commonly used in diaper manufacture. Additionally, such styrenic block copolymer based adhesives maintain adhesion in elastically demanding applications such as disposable diapers. Relatively low styrene content (from about 15% to about 24%) styrene-isoprene-styrene block copolymers are known to be useful for adhesives for such applications.
New improvements have necessitated the development of new hot melt adhesives for use in the manufacture of disposable diapers. Diaper manufacturers now use thinner gauge polyethylene to manufacture diapers. Not only does thinner gauge polyethylene produce a more esthetically pleasing diaper but it also represents significant cost savings. However, as the gauge of the polyethylene is decreased, the likelihood of burn through of these thin layers has increased. In the past, the lower limit on the operating temperature has been defined by the temperature at which the hot melt adhesive could be applied with sufficiently low viscosity for even and proper application. Such temperatures were in the range of 300.degree. F. to 350.degree. F. The industry would like to operate at from about 250.degree. F. to about 300.degree. F.
Thus, it is clear that there is a need for a hot melt adhesive which is characterized by sufficiently low viscosity for proper application in diaper manufacture at these lower temperatures to help prevent burn through of the thinner gauge polyethylene used in the new diaper assemblies. There are new commercial products which purport to satisfy this need. However, these commercial products do not provide the balance of adhesive performance properties which is provided by the present invention, i.e., a melt viscosity of less than 3100 cps at a temperature of 350.degree. F. and a shear adhesion failure temperature (SAFT) of greater than 71.degree. C. The present invention provides block copolymer compositions which can be used to make adhesives satisfying these criterion.